The present invention relates to a braking system for electric railcars operating near the stopping speed of the electric railcar.
Generally, to minimize required maintenance it is desirable to slow an electric railcar using an electric brake (ideally to a stop). Further, to simplify and save energy, it is advantageous to conduct electric braking using a regenerative braking system.
However, in order to complete stopping in a prior-art electric railcar, denoted at 12 in FIG. 13 the brake system is switched to pneumatic braking at about 5 km/hr., denoted at 11 in FIG. 3. Further, to achieve the same stopping distance, the brake system employs a switching system which decreases regenerative braking from about 10 km/hr in order to match the response time of the pneumatic brake.
In order to increase the electric braking range of a non-adhesion drive when the electric motor employs a linear motor, devoted by 10 in FIG. 3 the brake system employs a system for raising the inverter frequency by switching the phase order of the inverter at 0 Hz to a reverse phase range thereby applying a braking force of 100% by the electric brake until the railcar has stopped.
FIG. 2 is a block diagram showing, for example, speed control during braking of a linear motor electric railcar disclosed on pages 347 to 351 of "24th Domestic Symposium Theses Utilizing Cybernetics in Railway" (issued by Japan Railway Cybernetics Council in February, 1988).
In the drawing, a speed detector 1 is attached to the end of an axle or the like for detecting the speed of a linear motor electric railcar. A pattern generator 2 generates various types of control patterns in response to a command given from a drive cab or the like and outputs a slip frequency pattern f.sub.s in a speed control range. A frequency arithmetic unit 3 subtracts the slip frequency pattern f.sub.s outputted by the pattern generator 2 from a speed (rotor frequency) f.sub.r outputted by the speed detector 1 and outputs an inverter frequency f.sub.INV. A comparator 4 compares the inverter frequency f.sub.INV with a phase order switching reference frequency of 0 H.sub.z. A phase order determining circuit 5 and an absolute value comparing circuit 7 output to an inverter unit 6 a rotating direction command from the comparator 4. A braking force computing circuit 8, given a torque command from a driver cab or the like and a generated electric braking force T.sub.E from the inverter unit 6, outputs a signal of insufficiency. A pneumatic braking unit 9 applies a pneumatic braking force in response to the output of the braking force computing circuit 8.
Next, the operation or the prior-art linear motor electric railcar will be explained. The pattern generator 2 generates a slip frequency pattern f.sub.s according to a command from the driver cab, load conditions or the like. The frequency arithmetic unit 3 subtracts the slip frequency pattern f.sub.s from the speed (rotor frequency) f.sub.r detected by the speed detector 1 during braking as shown in an equation (1), and outputs and inverter frequency command value f.sub.INV. EQU f.sub.INV =f.sub.r -f.sub.s ( 1)
The fINV signal is applied through the absolute value computinq circuit 7 to the inverter unit 6. When the railcar is decelerated to nearly a stop, the f.sub.r in equation (1) is reduced, and there is a point at which the f.sub.INV becomes less than "0". In other words, there is a range the frequency f.sub.INV changes in sign from positive to negative. Then, a braking force is obtained by consuming energy at the secondary side of the electric motor by forming a magnetic field of a reverse phase of the rotating direction (advancing direction) of the electric motor. Therefore, it is necessary to replace the phase order of the output of the inverter so as to form the magnetic field of the reverse phase.
The comparator 4 compares the 0 Hz of the reference frequency with the output f.sub.INV of the frequency arithmetic unit 3, and outputs a phase order switching command to the phase order determining circuit 5 when the condition (2) below is met. EQU f.sub.INV .ltoreq.0 (2)
The phase order determining circuit 5 combines the rotating direction command and the output from the comparator 4, and determines the output phase order of the inverter and outputs it to the inverter unit 6.
As described above, the inverter frequency command value .vertline.f.sub.INV .vertline. and the inverter output phase order are applied as speed control commands to the inverter unit 6, thereby controlling the speed of the electric motor.
On the other hand, the inverter unit 6 outputs a generated electric braking force T.sub.E to the braking force computing circuit 8. The braking force computing circuit 8 computes the insufficiency of the torque command T.sub.p applied from the driver cab or the like by the following equation (3), and outputs a command T.sub.a to the pneumatic braking unit. EQU Ta=T.sub.p -T.sub.E ( 3)
The pneumatic braking unit thus applied the braking force obtained by the equation (3) so that the deceleration of the railcar becomes the desired value. However, since the pneumatic braking unit 9 needs a response time, the equation (3) cannot be instantly satisfied.
Since the prior-art braking system for the electric railcar is controlled as described above, the approximate speed of 0 km/hr cannot be detected due to the inaccuracy of the detector. Thus, there is a possibility that the electric railcar might be reversely driven due to the negative frequency. Further, since the output frequency of the inverter cannot be set completely to 0 Hz at the time of switching the phase order, there are variations in the frequency near 0 Hz, causing variations in the torque which drop the deceleration of the railcar. Thus, there are problems, such as failure in the riding comfort of the railcar.